The greatness of the Four Quartets lies partly in their abstract considerations, but also in the way that they are so particular in their imagery. They are poems of long walks in the English countryside, and boating off the north-eastern US and of London in gloomy threatened peace and the dust and smoke of war; they are poems of middle age and a sense of fading powers. They are at once an attempt at making a final general statement about the spiritual life and an intense last flowering of the poetry of a very specific person.

Eliot almost entirely abandoned poetry after Little Gidding and turned, for good and ill, to the theatre; I don't propose to consider his plays in this series. There is a paradox here – Eliot talks of faring forward, or not ceasing from exploration, but these four great poems are a total statement after which there was nothing much left for him to say.

This is partly because of their essentially musical structure – in which themes are constantly recapitulated in major and minor ways. The ghost children who never were get their major moment in Burnt Norton but are reprised in a few lines of Little Gidding; other poems of Eliot engage in metonymy of other texts, and so do these, but here echoes of other poems in the sequence are even more important. Not only is Eliot telling us that "all time is eternally present", that "in my end is my beginning", he is constantly showing us this as a matter of technique. Further, by implication, he is saying that the destiny of our souls is constantly constructed and reconstructed by meditations that are an echo chamber both of what is real and what might have been.

In these poems, then, we get echoes of Julian of Norwich and that other Thomas Elyot who wrote The Book of the Governor; we get "I sometimes wonder if this is what Krishna meant" teetering near pretentious self-parody but clearly needed; we get passages that modernise the piety of the metaphysicals, whose advocate Eliot had been "the wounded surgeon plies the steel" and the almost Celtic twilight "the boarhound and the boar/pursue their pattern as before/but reconciled among the stars". There's a sort of ecclesiastical tourism going on – he is shoring up fragments again but with a view to curing his soul this time.

These are poems of self-examination and regret and a sense of the need for penance: "The rending pain of re-enactment/of all that you have done, and been; the shame/of motives late revealed/of things ill done, and done to other's harm." It may be impossible to be any harder on Eliot than he was, some of the time and that the best of him, on himself.

But for all their obsession with Anglicanism as a living tradition, the Four Quartets are only occasionally a way of talking about the spiritual, specifically Christian.

As you would expect from the descendant of New England Puritans, there is a deep consciousness here both of personal sin and original sin but salvation is seen in terms of personal annihilation as much as purgation "consumed by either fire or fire". Christ is "the wounded surgeon" but not otherwise here; Eliot feels the deep guilt of the redemptive sacrifice of incarnate flesh but not the joy of birth or resurrection. Christians have been so glad of having such a great poet as a recruit that they have sometimes not noticed his less-than-entire orthodoxy.

These are poems, or rather one long great poem in many segments, about old age and about regret and about hope for some sort of mystical annihilation of a redeemed self; they are also poems about art and poetry and structure. This is not a sneer at the expense of the belief with which Eliot struggled and his attempt to make right in a spiritual realm what could not be fixed in this world – a poet cannot but conceive of the building and correction of a soul save through poetic structure.

Eliot created the symphonic self-recapitulating structure of the Four Quartets as a way of modelling what he hoped was possible in a religious sense; without sharing his belief, we can see these poems as magnificent in the purely aesthetic sense that he hoped he had moved beyond. If we believe that the making of art is one of the highest justifications of human life, it is possible to be consoled and moved and perhaps ennobled by these poems. Eliot might have hoped for more, but that's something.